NSW Department of Environment and Conservation, 2005
ISBN: 1 7412 2145 5
4 Taxonomy and Description
I. trypherus belongs to the Brassicaceae family, a family of 375 genera (3200 spp.) worldwide; with 52 genera (160 spp.) in Australia, many of which were introduced (Hewson 1982). There are three species in the genus Irenepharsus, all of which are endemic to Australia. Irenepharsus phasmatodes is restricted to Kangaroo Island in South Australia (Hewson 1982) while Irenepharsus magicus (Elusive Cress) has been recorded from East Gippsland in Victoria and in the Australian Alps (NPWS 2001).
I. magicus is known from just one collection in NSW and is listed as an endangered species on Schedule 1 of the TSC Act. It is one of the subject species of the Recovery Plan for the “lost” threatened flora of south-eastern NSW (NPWS 2001).
I. trypherus is a glabrous (hairless) annual or short- lived perennial herb. It has a spreading to erect habit and is often multi-stemmed, with stems growing to 250 cm. The stem leaves are simple, petiolate, thin, mostly 4 to 8 cm long, linear to lanceolate to elliptic in shape, with serrated margins. Its inflorescence is an elongated raceme with petal-like sepals, 2 to 2.5 mm long and white petals, 3 to 4 mm long. The fruit is a linear siliqua (dry, dehiscent pod), 5 to 20 mm long and 0.7 to 1 mm wide, which is curved and ascending. The seeds are oblong-ovoid, 0.7 to 1 mm long, and sticky when wet (Hewson 1982; Retter & Harden 1990; M. Bremner, DEC, pers. comm.).
I. trypherus can be distinguished from other species in the family Brassicaceae that occur in NSW by the following key features (Hewson 1982; Retter & Harden 1990):
- The entire plant is glabrous;
- Basal leaves are absent and the lower stem leaves are not lobed;
- Fruit is a siliqua, less than 1 mm wide and greater than 10 mm long with no beak (although the style may be persistent); and
- Seeds are sticky when wet, with an incumbent radicle and wings absent.
The species is superficially similar to the weed species Ageratina riparia (Mistflower), with which it co-exists at a few sites. The flowering period of I. trypherus however, extends from December to June while Mistflower flowers between June and November.